Add flake8 tests and fix flake8 notifications

Use latest flake8.

As a part of flake8 fixes:
- fix node_username in fuel_health/savanna.py,
- fix --tests option in fuel_plugin/ostf_client/ostf.py.

Change-Id: I3f8b667cf73e6426300febdb476bd2a312ba5baa
Closes-Bug: #1321907